The invention relates to rotatable flexible shafts and particularly to a flexible shaft capable of driving high torque loads with the shaft at unusual angles and with minimum backlash.
Conventional flexible shafts comprise an outer casing of flexible material and an inner rotational power transmission shaft formed by either (1) wrapped successive layers of wire around a central core wire or by (2) a conventional chain. In the first mentioned shaft, the successive layers of wire are wound in opposite directions with the outermost layer determining the direction of rotation in which the shaft is designed to rotate. If rotated in the opposite direction, the outer layer and the alternate under layers tend to unwind and become loosened, resulting in a reduced torque handling capability of the shaft. Conversely, as the shaft is rotated in the direction for which it was designed, the layers tend to become tight, increasing the torque handling capability but reducing the flexibility
In the second mentioned shaft, chain cores made of conventional links have medium torque handling capability with low flexibility provided the chain is kept under tension. If the chain tension is not maintained and becomes only slightly loose in the tubular shell of the casing, it will kink and form knots. Continued rotation of a knotted chain will soon tear apart the casing.
U.S. Pat. No. 5,554,073, overcomes the problem of chain knotting within the tubular shell by forming each link of the chain with two attached loops at a right angle to each other and each having a diameter that fits within the bore of the outer cashing. Each loop in the link therefore holds the other loop along the diameter of the shell and prevents misalignment and resulting knotting if the chain becomes loose.
Torque load capability of any drive shaft is a function of its material strength and its construction. A shaft made of copper or a readily frangible material will not have as long a life as a shaft made of strong steel.
Backlash, or the lost rotation between entry and exit of a shaft being reversed, is a problem in flexible shafts having chain cores when reversing is required because there is a considerable amount of backlash between each successive pair of links in the chain.
It has been found that a shaft having a single link construction with specially designed links will perform as well as the two loop chain described in U.S. Pat. No. 5,554.073 if the links are prevented from tumbling, that is, remain erect along the diameter of the tubular outer shell so that the links cannot tip forward or backward along the axis of the tubing.
The flexible shaft of the invention includes a flexible tubular outer casing containing a flexible shaft formed of a plurality of interconnected links of a design that prevents tumbling and knot-forming and permits unusually high torque loads with less than the usual amount of backlash. This design eliminates the right-angled connection of the two loop link of the above-mentioned patent and facilitates ease of manufacture of various sized links but also results in a stronger, simpler, more durable low cost product. It is contemplated that the flexible shafts to be described shall be constructed of steel or some equally strong composite for heavy duty, high torque loads.